FIG. 15 is a block diagram illustrating a configuration of a conventional synchronization detection device provided in a digital broadcast receiver. A synchronization detection device 100 illustrated in FIG. 15 includes a field synchronization detection section 101, a pattern match section 102, a count section 103, a backward guard level storage section 104, a forward guard level storage section 105, a synchronization determination section 106, and a synchronization detection signal generation section 107. The synchronization detection device 100 is provided in a digital demodulation system of a receiver compliant with the US digital terrestrial broadcasting standard. The VSB (Vestigial Side Band) scheme is employed as the modulation scheme for the digital demodulation system. The synchronization detection device 100 detects two kinds of synchronization signals (a field synchronization signal and a segment synchronization signal) from an input signal DT having a frame structure compliant with the ATSC (Advanced Television Systems Committee) standard, and outputs a field synchronization detection signal FS and a segment synchronization detection signal SS.
In FIG. 15, the field synchronization detection section 101 detects a field synchronization signal contained in the input signal DT, and outputs the field synchronization detection signal FS. Elements other than the field synchronization detection section 101 together form a segment synchronization detection section 109. The segment synchronization detection section 109 has a synchronization backward guard function and a synchronization forward guard function. The backward guard function refers to a function of transitioning from a synchronization-unestablished state to a synchronization-established state when a pattern match operation with a synchronization signal succeeds a predetermined number of times (referred to as “backward guard level”) consecutively in a synchronization-unestablished state. The forward guard function refers to a function of transitioning from a synchronization-established state to a synchronization-unestablished state when a pattern match operation with a synchronization signal fails a predetermined number of times (referred to as “forward guard level”) consecutively in a synchronization-established state.
The pattern match section 102 performs a pattern match operation between the input signal DT and the segment synchronization signal, and outputs a match result signal 202 indicating success or failure of the match operation. The count section 103 uses a counter provided therein to count the number of times a pattern match operation with the segment synchronization signal succeeds consecutively in a synchronization-unestablished state, and the number of times a pattern match operation with the segment synchronization signal fails consecutively in a synchronization-established state. The backward guard level storage section 104 stores a backward guard level 204 as described above, and the forward guard level storage section 105 stores a forward guard level 205 as described above.
The synchronization determination section 106 determines whether it is a synchronization-established state or a synchronization-unestablished state based on a count value 203 of the count section 103, the backward guard level 204 and the forward guard level 205. More specifically, the synchronization determination section 106 determines a transition to a synchronization-established state when the count value 203 becomes equal to or greater than the backward guard level 204 in a synchronization-unestablished state, and determines a transition to a synchronization-unestablished state when the count value 203 becomes equal to or greater than the forward guard level 205 in a synchronization-established state. A synchronization state signal 206 outputted from the synchronization determination section 106 is supplied to the count section 103 and the synchronization detection signal generation section 107.
The synchronization detection signal generation section 107 outputs the segment synchronization detection signal SS when it receives the match result signal 202 (whether it indicates a match success or a match failure) in a synchronization-established state.
However, a VSB demodulation system with the above-described conventional synchronization detection device provided therein has the following problems. When some object exists near the antenna of a digital broadcast transceiver, a VSB demodulation system receives both the original radio wave and a reflected wave that is reflected off the object, whereby the image is not displayed at all on the screen of the receiver. This phenomenon is called a “near-ghost”. When the receiver receives a radio wave such that a near-ghost occurs, the segment synchronization pattern is disturbed (see FIG. 9) in the input signal to the VSB demodulation system. Therefore, a pattern match operation only for the upper one bit (sign bit) between the input signal and the segment synchronization signal may result in a failure to properly detect a synchronization signal or a detection of a synchronization signal at an erroneous timing.
Moreover, the segment synchronization detection signal SS outputted from the synchronization detection device is inputted to a waveform equalization section that is provided in a stage subsequent to the synchronization detection device. However, when the segment synchronization signal is detected at an erroneous timing, the waveform equalization section can only function to ensure a proper timing for the segment synchronization detection, and can no longer perform its primary function of removing a ghost caused by a transmission path.
Thus, it is an object of the present invention to provide a synchronization detection device having a desirable synchronization detecting capability and/or a desirable synchronization maintaining capability, and to provide a VSB demodulation system having a high near-ghost interference removing capability.